1. Field of the Invention
This invention relates to new water-dispersible plastics precursors which are solid at temperatures below about 40.degree. C., to a process for the production of stable, aqueous plastics dispersion, particularly aqueous polyurethane-polyurea dispersions, and to the use of the plastics precursors as cross-linking agents for plastics dispersed in water.
2. Description of the Prior Art
Processes for the production of stable, aqueous polyurethane-polyurea dispersions are known (cf. for example German Pat. Nos. 1,184,946 and 1,178,586; German Auslegeschrift No. 1,237,306; German Offenlegungsschrift Nos. 1,495,745; 1,595,602; 1,770,068; 2,019,324; 2,314,512; 2,446,440; 2,811,148; and also D. Dieterich et al., Angew. Chem. 82., 53 (1970)). The described dispersions are based on the principle of incorporating hydrophilic centers into a macromolecular chain of a polyurethane-(polyurea) molecule. In the known dispersions, these hydrophilic centers or so called "internal emulsifiers" are ionic groups or ether functions. The groups are either incorporated into the prepolymer or are used as modified amines for chain-extending the prepolymers which contain at least two terminal NCO-functions.
The hitherto known dispersions may be produced by various processes, as described, for example, by D. Dieterich and H. Reiff in Angew. Makromol. Chemie 26, 85 (1972). In general, either a solution of a polyurethane in an organic solvent is converted into an aqueous dispersion or a prepolymer stage is dispersed in liquid form in water in the presence or absence of solvents. For example, a liquid prepolymer ionomer containing NCO-groups may be introduced into water with vigorous stirring, resulting initially in the formation of an emulsion of the prepolymer which further reacts by chain-extension with water or with a diamine or higher polyamine to form the high molecular weight polyurethane urea.
One particularly simple method of dispersion is described in German Offenlegungsschrift No. 1,913,271 (U.S. Pat. No. 3,756,992). In this method, a solid or liquid polyurethane polyelectrolyte capable of the addition of formaldehyde to form methylol groups is dispersed by mixing with water and is converted into a polyurethane polyelectrolyte containing methylol groups by the addition of the formaldehyde or formaldehyde derivatives. The thus-obtained polyurethane polyelectrolyte is condensed to completion in the dispersion or even on a substrate to form the high molecular weight polyurethane.
For dispersion, water is added to the stirrable melt until the water forms the continuous phase, the stage of a water-in-oil (W/O) emulsion generally being passed through to begin with. It is also possible to prepare a (W/O) emulsion at elevated temperatures, the thus-prepared emulsion changing into an oil-in-water (O/W) emulsion on cooling. The dispersion is almost always prepared at elevated temperature, preferably at temperatures of from 50 to 120.degree. C. This is necessary, on the one hand, because the prepolymer stage to be dispersed has excessive viscosity at room temperature, with the result that expensive apparatus, such as screw extruders, would be necessary for dispersion, and, on the other hand, because the dispersion process generally takes place more quickly, the higher the temperature applied. The dispersion of solvent-free melts of NCO-prepolymers is also most always carried out at elevated temperature, the residual isocyanate reacting off with water and/or added amine.
Conventional ionomer dispersions generally show inadequate frost stability, so that storage and transportation in winter months are expensive because heated storage spaces are required. Apart from this, the transport of aqueous dispersions over considerable distances is basically unsatisfactory because large quantities of water have to be transported at great expense.
U.S. Pat. Nos. 4,094,842 and 4,123,423 describe possible methods of preparing the aqueous dispersions by the spontaneous dispersion of NCO-free solids in water and by the addition of any cross-linking additives which may be necessary at the consumer's end. The polymers, particularly polyurethanes, produced by the processes described in these publications are predominantly cross-linked products. However, it would be comparably desirable to produce predominantly linear products which are more suitable for numerous applications by virtue of the higher quality level thereof. However, these products would also have to be transported in solid form and only converted into an aqueous solution or dispersion at the consumer's end in order to make full use of this economic advantage.
It has now surprisingly been found that certain NCO-group-containing plastics precursors described in more detail below spontaneously change into a stable dispersion on contact with water at temperatures below the softening range thereof, and, after dispersion, may be reacted with suitable chain-extending agents or cross-linking agents to form high molecular weight plastics. Accordingly, it is possible to introduce these plastics precursors into an aqueous medium either in particulate form or (coarse) powder form or to pour an aqueous medium over them and thus to quasi "dissolve" them to form dispersions, the reactive NCO-groups remaining substantially intact.
These properties of the plastics precursors according to the present invention as described below are surprising because isocyanate groups are generally highly reactive to water, particularly when the isocyanate is present in a sufficiently hydrophilic form to ensure dispersibility in water. Thus, it is known that, when liquid NCO-prepolymers are slowly mixed with water, chain-extension and then cross-linking rapidly occur, accompanied by gelling, which, of course, makes it impossible to produce useful plastics dispersions. Since the "dissolution" process by which the dispersion is formed takes place relatively slowly in the case of solid NCO-group-free prepolymers according to the publications mentioned above, it had also been expected that, if an attempt was made to "dissolve" the solid in water to form a dispersion, the free isocyanate groups in the plastics precursors according to the present invention would react off with the water so quickly that the formation of a plastics dispersion would be prevented by premature extension or cross-linking.